1. Field of the Invention
This invention relates to low noise optical amplifiers for fiber optic transmission systems, and more particularly to low noise discrete, distributed and hybrid Raman amplifiers for broadband communication systems.
2. Description of Related Art
Stimulated Raman scattering is an important nonlinear process that turns optical fibers into amplifiers and tunable lasers. Raman gain results from the interaction of intense light with optical phonons in silica fibers, and Raman effect leads to a transfer of energy from one optical beam (the pump) to another optical beam (the signal). The signal is downshifted in frequency (or upshifted in wavelength) by an amount determined by vibrational modes of silica fibers. The Raman gain coefficient gr for the silica fibers is shown in FIG. 1. Notably, the Raman gain gr extends over a large frequency range (up to 40 THz) with a broad peak centered at 13.2 THz (corresponding to a wavelength of 440 cmxe2x88x921). This behavior over the large frequency range is due to the amorphous nature of the silica glass and enables the Raman effect to be used in broadband amplifiers. The Raman gain also depends on the composition of the fiber core and can vary with different dopant concentrations.
Raman amplification has some attractive features. First, Raman gain is a good candidate for upgrading existing fiber optic links because it is based on the interaction of pump light with optical phonons in the existing fibers. Second, there is no excessive loss in the absence of pump power, other than the loss of the fiber insertedxe2x80x94an important consideration for system reliability.
Cascading is the mechanism by which optical energy at the pump wavelength is transferred, through a series of nonlinear polarizations, to an optical signal at a longer wavelength. Each nonlinear polarization of the dielectric produces a molecular vibrational state corresponding to a wavelength that is offset from the wavelength of the light that produced the stimulation. The nonlinear polarization effect is distributed throughout the dielectric, resulting in a cascading series of wavelength shifts as energy at one wavelength excites a vibrational mode that produces light at a longer wavelength. This process can cascade through numerous orders. As an example, cascade Raman orders for different pump wavelengths are illustrated in FIG. 2. Because the Raman gain profile has a peak centered at 13.2 THz in silica fibers, one Raman order can be arranged to be separated from the previous order by 13.2 THz.
Cascading makes stimulated Raman scattering amplifiers very desirable. Raman amplification itself can be used to amplify multiple wavelengths (as in wavelength division multiplexing) or short optical pulses because the gain spectrum is very broad (a bandwidth of greater than 5 THz around the peak at 13.2 THz). Moreover, cascading enables Raman amplification over a wide range of different wavelengths. By varying the pump wavelength or by using cascaded orders of Raman gain, the gain can be provided over the entire telecommunications window between 1300 nm and 1600 nm.
Raman gain can be used in both discrete and distributed amplifiers. The main advantages of distributed Raman amplification are that the effective noise figure (NF) is improved and existing systems can be upgraded. Intuitively, the NF improves because the signal is continuously amplified and never gets too weak. The additional system margin allowed by distributed amplification can be used to upgrade the system speeds, increase the spacing between amplifiers or repeaters, or to handle the variability in fibers for installed systems. When using distributed amplification, the pump light can be counter-propagating to the signal direction. Simulations and experiments have shown the improvement in noise figure achieved using distributed amplification. For example, a calculation from first principles for a chain of optical amplifiers shows the improvement in signal-to-noise ratio (SNR) for more closely spaced amplifiers. The case of purely uniform amplification gives an improvement of about NF=2 dB compared with amplifiers spaced evenly every 21.7 km and an improvement of about NF=4 dB compared with amplifiers spaced evenly every 43.4 km (where NF (dB)=SNRIN (dB)xe2x88x92SNROUT (dB)).
Experiments have also verified the improvement in NF performance for distributed amplification. For instance, experiments in a 514 km Raman amplifier chain have shown an improvement in noise performance of 2 dB compared with a similar amplifier chain using lumped EDFA""s spaced roughly every 45 km. This is less than the ideal case because the pump light attenuates along the length of the fiber, leading to periodic but non-uniform amplification. In addition, a combination of distributed Raman amplification and EDFA""s has been used to extend the repeater spacing to 240 km for a 5280 km WDM 8-channel system. The performance demonstrated in this experiment was comparable to that of a system of similar length and capacity using conventional EDFA""s spaced by 80 km. Therefore, the additional NF margin from distributed amplification can be used to significantly increase the repeater spacing of long-haul transmission systems. Furthermore, a distributed Raman amplifier is tested in a 45 km length of transmission fiber that is pumped by two pumps at 1453 nm and 1495 nm. The resulting transparency gain bandwidth is 92 nm, and the Raman amplifier is shown to perform better than a lumped EDFA with a NF equal or higher than 5 dB.
Another use of hybrid or distributed amplifiers is to reduce nonlinearity impairments from four-wave mixing (4WM) and Raman gain tilt that become increasingly important as new bands are added and the channel count increases. One way of minimizing these nonlinearity impairments is to reduce the power per wavelength channel. This can be achieved without degradation of the signal-to-noise ratio at the receiver by using hybrid or distributed Raman amplification. In particular, distributed Raman amplification can be achieved by pumping the fiber composing the transmission line with a Raman oscillator or laser diodes directly. The pump light produces Raman gain for the signal using the inherent Raman gain in the transmission fiber. Since the gain is inherent to the transmission line, this provides a graceful means of upgrading even existing fiber-optic systems.
The power per channel can be reduced because distributed Raman amplification cancels or compensates for the loss in the fiber. Said another way, the distributed Raman gain has an effectively better noise figure than its discrete amplifier counterparts. The channel power can be lowered to the point that nonlinearities become insignificant. For example, in a typical transmission system at power of 0 dBm (1 mW) is used at OC-48 or 2.5 Gb/s and 6 dBm (4 mW) at OC-192 or 10 Gb/s per channel. With the addition of distributed amplification, OC-192 systems have been demonstrated in the laboratory with power per channel as low as xe2x88x9213 dBm (0.05 mW).
Distributed Raman amplification can also help in gain control or gain clamping, i.e., It is undesirable to have the gain level change when channels are added or dropped, such as when optical add/drop multiplexers are used. This gain clamping problem can be solved to a large extent by using distributed Raman amplification because the power per channel is significantly reduced. The lower power insures that there will be negligible gain or pump depletion. Therefore, the combination of lower power per channel and negligible gain depletion provides an effective gain clamping.
That nonlinear effects in fiber transmission systems can be avoided by use of distributed or hybrid Raman amplification has been illustrated in a number of recent experiments. Transmission in DSF around the zero-dispersion region in a single wavelength band has been demonstrated. Dense-WDM (DWDM) transmission of 32 channels with 50 GHz spacing and bit-rate of 10 Gb/s over 8xc3x9780 km has been demonstrated. Normally, DWDM systems in the neighborhood of the zero dispersion wavelength suffer from 4WM penalties. However, by lowering the channel power down to xe2x88x9213 dBm with the use of distributed Raman amplification, 4WM can be avoided and the results verify feasibility of DWDM transmission in DSF.
DWDM transmission near the zero dispersion wavelength without 4WM or other nonlinearity penalties in a single wavelength band has been demonstrated in multiple experiments: (a) 25-channel, 100 GHz spacing of 10 Gb/s channels, transmission over 8xc3x9783 km; (b) 49-channel, 50 GHz spacing of 10 Gb/s channels, transmission over 4xc3x9783 km. Significant improvements can be obtained at a pump power of only 440 mW in DSF by using hybrid Raman/erbium-doped fiber amplifiers.
Distributed Raman amplification can minimize nonlinear effects between WDM channels effectively in single band experiments (i.e., in the so-called xe2x80x9cC-bandxe2x80x9d or conventional band between 1535 and 1565 nm).
One benefit of DRA""s, such as reduction of the nonlinear effects among WDM signal channels, can increase the bandwidth utilization efficiency in WDM systems of some embodiments. Since 1996, when WDM systems were first commercially introduced, the number of wavelength channels has increased dramatically (FIG. 27). For example, state-of-the-art systems in 1999 have more than 100 wavelength channels. Given this rapid increase in channel count, the question is how to achieve the next decade increase in number of channelsxe2x80x94or, ultra-dense-WDM (U-DWDM) systems. What are the key enabling technologies for systems of 1000 or more wavelength channels?.
There are fundamental limitations to achieving 1000+ wavelength systems. First, as the density of channels increases, nonlinear interactions between channelsxe2x80x94such as four-wave-mixing and Raman gain tiltxe2x80x94can limit the system performance. The second problem of U-DWDM systems is the transmitter complexity. For example, if temperature and frequency stabilized LD""s are used as the light source, then as the density of channels increases, it can become increasingly more difficult to reduce the channel spacing. Moreover, as the number of channels increases, the footprint or physical size of the transmitter becomes increasingly large. A third challenge of U-DWDM systems is the filtering technology. Filters with high-contrast and narrow channel spacing are required to place many channels in close proximity to one-another.
One illustration of the fundamental fiber nonlinearity limits can be taken from a systems viewpoint. The parameters used in one study are: fiber link of 30 km, 1550 nm wavelength, loss of 0.2 dB/km, Aeff=50 xcexcm2, channel spacing of 10 GHz, for standard fiber chromatic dispersion of 16 ps/nm-km or for DS fiber a chromatic dispersion of 1 ps/nm-km. FIG. 28a shows the four-wave-mixing efficiency as a function of channel spacing at 1550 nm. The solid curve represents standard fiber, while the dashed curve represents DS fiber. The four-wave-mixing efficiency can be much higher in DS fiber because the phase-matching can be more readily achieved in low dispersion fiber. FIG. 28(b) shows the maximum power per channel versus the number of channels that ensures stimulated Raman scattering (SRS), carrier-induced phase modulation (CIP), stimulate Brillouin scattering (SBS), and four-photon mixing (FPM) degradations are below 1 dB for all channels. For standard or DS fiber, channel spacing on the order of 10 GHz can be problematic. For a number of channels approaching 1000, Raman gain tilt can be the first nonlinearity to affect the system, and the power per channel can approach xcx9c0.01 mW/channel (xe2x88x9220 dBm) to avoid nonlinearities.
Hybrid amplifiers using DRA""s can serve as an enabler for 1000+ wavelength systems. One benefit of DRA""s can be better NF, which can be used to lower the signal amplitudes so that nonlinear interaction between channels can be reduced. Nonlinear effects can be avoided using DRA""s. Despite operation near the zero dispersion wavelength, four-wave-mixing penalties can be avoided in 25-50 channel systems. The optical SNR improvement using DRA""s can be 6.6 dB in DS fiber and 7 dB in standard fiber. The input channel power can be reduced by this many decibels and still maintain the system SNR.
Hybrid amplifiers can improve system with improved NF. This NF improvement can be used as additional system margin to extend the spacing between amplifiers, lower signal powers to avoid fiber nonlinearity or increase the system bit-rate. However, these experiments focus on using DRA""s only in the so-called C-band (between 1530-1565 nm), where the discrete amplifier is an EDFA. Also, the experiments are made in DS fiber, where the zero dispersion wavelength coincides with about 1550 nm. One problem with implementing DRA""s in this wavelength range is that it will prevent further expansion of new low-loss windows later on.
There is a need for low noise Raman amplifiers and broadband transmission systems. There is a further need for distributed, discrete and hybrid amplifiers with improved noise figures. Another need exists for optical amplifiers suitable for wavelengths of 1480 nm or less where the loss of the fiber increases.
An object of the present invention is to provide a Raman amplifier with an improved noise figure.
Another object of the present invention is to provide a Raman amplifier with bi-directional pumping and an improved noise figure.
Yet another object of the present invention is to provide a Raman amplifier with bi-directional pumping and an improved noise figure, where the bi-directional pumping is achieved by using a pump to amplify the signal in a counter-propagating manner.
A further object of the present invention is to provide a Raman amplifier that is bi-directionally pumped and includes at least a first and a second pump as well as at least one additional pump that co-propagates with the signal to amplify the first pump.
Yet a further object of the present invention is to provide a Raman amplifier with multiple orders of Raman pumps that are bi-directional, includes at least a first and a second pump and at least one additional pump that co-propagates with the signal to amplify the first pump.
Another object of the present invention is to provide a bi-directionally pumped Raman amplifier with pump modules that can be multi-wavelength Raman oscillators, single wavelength Raman oscillators, laser diode pumps and combinations thereof.
Still another object of the present invention is to provide a low noise distributed Raman amplifier with bi-directional pumping.
Yet another object of the present invention is to provide a low noise discrete Raman amplifier with bi-directional pumping.
Another object of the present invention is to provide a low noise hybrid Raman amplifier with bi-directional pumping.
These and other objects of the present invention are achieved in a Raman amplifier assembly. A Raman amplifier is configured to receive a signal from a signal source. The signal travels in an upstream direction in the Raman amplifier. A first pump source is coupled to the Raman amplifier. The first pump source produces a first pump beam that travels in a downstream direction and is counter-propagating relative to the signal. A second pump source is coupled to the Raman amplifier and produces a second pump beam that travels in the upstream direction. The second pump source has an average relative intensity noise of less than xe2x88x9280 dB/Hz.
In another embodiment of the present invention, a multi-stage Raman amplifier apparatus includes a Raman amplifier configured to receive a signal from a signal source. The signal travels in an upstream direction in the Raman amplifier. A first pump source is coupled to the first Raman amplifier. The first pump source produces a first pump beam in a downstream direction that is counter-propagating relative to the signal. A second pump source is coupled to the first Raman amplifier. The second pump source produces a second pump beam that travels in the upstream direction. A third pump source is coupled to a second Raman amplifier. The third pump source produces a third pump beam that travels in the downstream direction.
In another embodiment of the present invention, a Raman amplifier assembly includes an optical transmission line with a first port and a second port. At least a portion of the optical transmission line produces Raman gain. A first pump source produces a first pump beam. The first pump beam and a first signal of multiple wavelengths enter the first port and travel in a downstream direction from the first port to second port. A second pump source produces a second pump beam. The second pump beam and a second signal of multiple wavelengths enter the second port and travel in an upstream direction from the second port to the first port. At least a portion of the second pump beam pumps the first pump beam, and at least a portion of the first signal wavelengths have shorter wavelengths than the second signal wavelengths.
In another embodiment of the present invention, a Raman amplifier assembly includes a Raman amplifier configured to receive a signal from a signal source. The signal travels in an upstream direction in the Raman amplifier. A first pump source is coupled to the Raman amplifier. The first pump source produces a first pump beam that travels in a downstream direction and is counter-propagating relative to the signal source. A second pump source is coupled to the Raman amplifier and has an average relative intensity noise of less than xe2x88x9280 dB/Hz. The second pump source produces a second pump beam that travels in the upstream direction. The second pump beam has wavelengths that are shorter than wavelengths of the first pump beam.
In another embodiment of the present invention, a method of broadband amplification provides a Raman amplifier assembly that includes an optical transmission line with a first port and a second port. At least a portion of the optical transmission line produces Raman gain. The Raman amplifier assembly is pumped with at least a first pump beam and a second beam. At least a portion of the second pump beam pumps the first pump beam. A first signal of multiple wavelengths is introduced into the first port. A second signal of multiple wavelengths is introduced into the second port. The first and second signals are amplified.
In another embodiment of the present invention, a method of broadband amplification provides a first pump source, a second pump source with an average relative intensity noise of less than xe2x88x9280 dB/Hz and a Raman amplifier assembly. The Raman amplifier assembly includes an optical transmission line with a first port and a second port. At least a portion of the optical transmission line produces Raman gain. The Raman amplifier assembly is pumped at the first port with at least a first pump beam, and at the second port with a second pump beam. The second pump beam has wavelengths that are shorter than wavelengths of the first pump beam. A signal is introduced into the second port. The signal is amplified.
In another embodiment of the present invention, a Raman amplifier assembly includes a Raman amplifier configured to receive a signal from a signal source. The signal travels in an upstream direction in the Raman amplifier. A first pump source is coupled to the Raman amplifier and produces a first pump beam. The first pump beam travels in a downstream direction and is counter-propagating relative to the signal. The first pump source is substantially depolarized. A second pump source is coupled to the Raman amplifier and produces a second pump beam. The second pump beam travels in the upstream direction and pumps the first pump beam.